The present invention is a method for the preparation of hydrophobic precipitated silicas which are useful as reinforcing fillers in rubber compositions. The method comprises two steps, where in the first step an aqueous suspension of precipitated silica is contacted with an organosilicon compound in the aqueous phase in the presence of a catalytic amount of an acid to effect hydrophobing of the precipitated silica thereby forming an aqueous suspension of a hydrophobic precipitated silica. In the second step the aqueous suspension of the hydrophobic precipitated silica is contacted with a water-immiscible organic solvent at a solvent to silica weight ratio greater than 5:1 to effect separation of the hydrophobic precipitated silica from the aqueous phase. In a preferred process the hydrophobic precipitated silica has a surface area within a range of about 100 m.sup.2 /g to 750 m.sup.2 /g.
Although hydrophobic precipitated silicas prepared by the present method are useful in many applications such as reinforcing and extending filler in natural rubbers, thermal insulation, and as filler in floatation devices, they are particularly useful as reinforcing fillers in silicone rubber compositions. It is well known that silicone rubbers formed from the vulcanization of polydiorganosiloxane fluids or gums alone generally have low elongation and tensile strength values. One means for improving the physical properties of such silicone rubbers involves the incorporation of a reinforcing silica filler into the fluid or gum prior to curing. It is known to use treated pyrogenic silica as a reinforcing filler in such silicone rubbers. However, pyrogenic silica has the disadvantage of being expensive. Therefore, it would be advantageous to replace at least a portion of the pyrogenic silica used in such applications with a precipitated silica.
Attempts to substitute precipitated silica for pyrogenic silica in curable silicone rubber compositions have not been entirely successful for a number of reasons. Precipitated silica has in common with pyrogenic silica the tendency to interact with the polydiorganosiloxane fluid or gum causing a phenomenon typically referred to as "crepe hardening". A great deal of effort has been made in the past to treat the surface of reinforcing silica fillers, both pyrogenic and precipitated silica, with organosilanes or organosiloxanes to make the surface of the silica hydrophobic. This surface treatment reduces or diminishes the tendency of the compositions to crepe harden and improves the physical properties of the cured silicone rubber. Processes for treating precipitated silica have typically been difficult to conduct on a commercial scale. The present method is one that can economically be conducted on a commercial scale to make hydrophobic precipitated silica.
Treblinger et al., U.S. Pat. No. 3,904,787, describe the treatment of a precipitated silica in aqueous suspension with an organohalosilane at a temperature within a range of 15.degree. C. to 70.degree. C. The hydrophobic precipitated silica is filtered, washed, dried, and tempered by heating in the range of 200.degree. C. to 500.degree. C.
Reinhardt et al., U.S. Pat. No. 4,015,031, describe a process where a precipitated silica in powdered form is heated to a temperature of about 200.degree. C. to 300.degree. C. with agitation to fluidize and then treated dropwise with an organosilane which is stable and boils below 300.degree. C.
Reinhardt et al., U.S. Pat. No. 4,072,796, describe a process where an acidic wet suspension of precipitated silica at a temperature of about 50.degree. C. to 90.degree. C. is hydrophobed with a prepolycondensed organohalosilane or a prepolycondensed mixture of organohalosilanes. The hydrophobed precipitated silica is filtered, washed, dried, and tempered at about 300.degree. C. to 400.degree. C.
Nauroth et al., U.S. Pat. No. 4,208,316, U.S. Pat. No. 4,273,589 and U.S. Pat. No. 4,308,074, describe the treatment of dried precipitated silica with organosilicon compound hydrophobing agents in a ratio of 10:0.5 to 10:3. The product obtained thereby is tempered for 60 to 180 minutes, preferably 70 to 130 minutes, at a temperature of 200.degree. C. to 400.degree. C.
Parmentier et al., U.S. Pat. No. 5,009,874, describe a method for making a hydrophobic, essentially spheroidal precipitated silica, useful as a reinforcing filler in silicone elastomers. In a first step the precipitated silica in aqueous suspension is hydrophobed with an organosilicon compound. In a second step a water immiscible organic solvent is added to effect separation of the hydrophobic precipitated silica from the aqueous phase. The water immiscible organic solvent is added to the process at a volume(L) to weight (Kg) ratio of silica of from 1 to 5 and preferably from 1.5 to 4.5.